Why sometimes (before a noun) is it just a "du" or just a "le or la" and sometimes both?
I understand that la is "the" and du is more or less a place holder for saying "it is a noun" but i just am confused on when and when not to use them, and certainly get confused when both are used.
This can be quite confusing, but understanding a few grammatical terms and concepts can help to clear up a bit of the confusion. As mentioned in another response, "du" is oftentimes a combination of the preposition "de" and the definite article "le" (de + le = du), but this is not always the case. There is another "du" which is called the partitive article and this is probably what you ran into in your lesson. The partitive article is used before nouns that cannot be counted, such as liquids, and it is often translated as some. This can be confusing, however, because in English we use the word "some" for both count and non-count nouns. For example, we say "I eat some apples" and "I eat some rice". In French, however, you would say "Je mange des pommes" (using the indefinite article "des" because apples can be counted) but you would say "Je mange du riz" (using the partitive article "du" because rice is considered a non-count noun).
While you are correct in saying that articles are placeholder indicating that "this is a noun", they also communicate more subtle information. Think of the difference between saying "I see the train" (definite article) and "I see a train" (indefinite article). Definite articles express that the noun being referred to is known by both the speaker and the listener. It is a specific instance of that noun that both parties are familiar with. The indefinite and partitive articles indicate that either the speaker and/or the listener is not familiar with the noun being referred to, and thus it is a general instance of that noun. Again, the difference between the indefinite and the partitive is that one is used with countable nouns and the other with non-countable nouns.
That's probably a lot more of an answer than you bargained for, but I hope it helps. Articles are tricky. On one hand they are very basic and one of the first things we learn, but they are also very subtle and hard to master. I have included some links below to some useful explanations of some of the grammatical terms mentioned above:
http://www.laits.utexas.edu/tex/gr/det5.html http://www.laits.utexas.edu/tex/gr/det2.html http://www.laits.utexas.edu/tex/gr/det4.html http://www.laits.utexas.edu/tex/gr/pre1.html http://www.laits.utexas.edu/tex/gr/pre2.html